Implement Fold entirely in its own terms

[atifaziz]

This PR implements Fold entirely in its own terms; that is, without any dependency on or composing on top of AsserCount and Index. Fold is expected to be used for short sequences (up to a maximum length of 16 elements), so this has the positive effect of avoiding extra enumerator allocations and virtual dispatches.

[codecov]

Codecov Report

Merging #969 (0a0612d) into master (777aa24) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 0a0612d differs from pull request most recent head 430fe45. Consider uploading reports for the commit 430fe45 to get more accurate results

@@           Coverage Diff           @@
##           master     #969   +/-   ##
=======================================
  Coverage   92.59%   92.60%           
=======================================
  Files         113      113           
  Lines        3432     3436    +4     
  Branches     1049     1051    +2     
=======================================
+ Hits         3178     3182    +4     
  Misses        191      191           
  Partials       63       63           

Impacted Files	Coverage Δ
MoreLinq/AssertCount.cs	96.15% <100.00%> (-0.15%)	⬇️
MoreLinq/Fold.cs	100.00% <100.00%> (ø)
MoreLinq/Fold.g.cs	100.00% <0.00%> (ø)

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